Free piston engine



Jan. 20, H G SPIER FREE PISTON ENGINE 2 Sheets-Sheet 1 Filed Deo. ll,1957 ATTORNEYS Jam 20, 1959 H. G. SPIER 2,869,524

EREE PIsToN ENGINE 2 Sheets-Sheet 2 Filed Dec. ll, 1957 POWER GASD\SCHARGE INVENTOR.

BY @MEQW ArroRA/Efs United tates Patent i FREE PHSTUN ENGINE Hans G.Spier, Hamilton, hio, assigner to Baldwin- Linla-Hamiiton Corpnration,Hamilton, Ohio, a corporation of Pennsylvania Application December 11,1957, Serial No. 702,024

2 Claims. (Cl. 12S-46) This invention relates to free piston engines andhas for its primary object to provide a free piston engine havinggreater efficiency than known engines.

It is known that the heat rejection to the coolants and by radiation innormally cooled free piston power gas generators amounts to about `17percent in the power cylinder and about 7.5 percent in the powerpistons. It has been found that a specific fuel consumption of about0.350 poundsvper gas horsepower per hour or a thermal eflieiency of 38percent can be practically obtained with these levels of heat rejection,the remaining losses thus amounting to about 37.5 percent. Assuming thatthese internal power and heat losses of 37.5 percent are unavoidable inpractice, the present invention proposes to increase the v`eiiiciency ofa free piston power gas generator by eliminating or greatly reducing thepower cylinder and piston heat losses. It will be apparent that if theselosses, amounting to 24.5 percent of the heat in the cycle,

could be Vcompletely eliminated, the thermal efciency of the cycle wouldrise to 62.5 percent, and the specific fuel consumption would drop to0.22 lbs./ G. H. P./hr.

In accordance with the present invention, the power pistons and thepower cylinder are each substantially uncooled. These portions thenoperate at a mean ternperature approximately equal to or somewhat higherthan that of the gas discharged by the machine. Since piston ringcontact or any other contact of the hot pistons with the equally hotcylinder is impossible, the present invention provides means to guidethe pistons internally and maintains a small, free running clearancebetween the power pistons and the power cylinder in which theyreciprocate.

The invention provides a free piston power gas generator having numerousadvantages over known units, which advantages may be briefly enumeratedas follows: (l) higher gas pressures are permitted if desired, (2)higher thermal efficiencies are attainable, (3) the service life of Vanengine is extended by the elimination of piston rings, (4) oilconsumption is reduced, (5) firing pressures are increased for increasedoutput and efficiency, (6) accessory requirements are reduced, (7) theengine is less expensive to construct initially, and (8) the frictionlosses are reduced.

Other objects and advantages will become apparent from the followingdescription of a preferred embodiment of the invention, reference beinghad to the accompany ing drawings, in which- 4Figure 1 is a diagrammatic,side elevation, with parts broken away, of a free piston power gasgenerator incorporating the present invention;

Figs. 2, 3, and 4 are enlarged sectional views of a portion of a pistoncrown seal; and

Fig. 5 is a somewhat diagrammatic central vertical sectional view of oneend of the machine shown in Fig. l.

The drawings show a free piston power gas generator embodying thepresent invention. The machine is essentially symmetrical about avertical center line passing through the injector nozzle as is common inthe art so Fei-tentati Jan. 20, 1959 that the more detailed figures showonly one end. As is customary, the engine comprises a central powercylinder portion 1li and, on each end, large compressor cylinders 12 and14. The spaces at the outer end of the compressor cylinders, in theconfiguration shown, are used as bounce spaces 16 and 18 so that initialcompression ofl the scavenging and charging air takes place on theinward stroke of the pistons.

The power section of the pistons working in the power cylinders isdesignated 20 in each instance, and each of the powe'r sections isconnected by a trunk section 22 to a larger compressor piston 24 workingin its respective oneV of the compressor cylinders. As is customary,each larger section of the piston compresses air in the bounce spacebehind it during the outward stroke and thus stores up the energy neededfor the next succeeding inward stroke. Also on the outward stroke, acharge of fresh air is drawn into the compressor cylinders throughintake valves 30; on the inward stroke of each compressor piston thischarge of air is discharged through valves 32 to a scavenging airreceiver or space 345. in the form of the gas generator shown in thedrawings the scavenging air space 34 communicates with the bounce spaces16 and 1S, but this is not necessarily true in more conventionalmachines.

As is also customary, the intake and discharge ports for the powercylinder are opened and closed by the power pistons. The intake orscavenging ports are in the form of Va peripheral series designated 36,at one end of the power cylinder iii, and discharge ports 38 are in asimilar series at the opposite end. The spacing of thesev ports withrespect to the center line of the machine is such that the dischargeports open somewhat ahead of the intake ports, and remain open for thepassage of scavenging air through the power cylinder. The dischargepassage for power gas is designated i0 in the drawings and may beconnected to an appropriate turbine or other gas consumer in a mannerwell known in the art.

Fuel is injected into the power cylinder by a conventional injector 42.The fuel pump and other conventional accessories, as well as thesynchronizing means fcr the pistons have been omitted from the drawingsin the interests of clarity.

in the usual free piston power gas generator the power piston carries anumber of conventional piston rings, and the interior of the powercylinder is lubricated. Since the function of the machine is to suppiygas at elevated temperatures and pressures, the operating temperature ofthe cycle is kept as high as possible commensurate with satisfactoryoperation. The high temperatures burn the lubricating oil from thecylinder walls with the result that piston ring failure is the mostcommon form of breakdown of a free piston machine.

The present invention supports and guides the pistons on inwardlyextending tubular members Sii which are a part of the frame structure ofthe machine and are bolted or otherwise ixed to the end walls or framemembers. These tubular guide members are s-o proportioned and designedthat a negligible distortion occurs at operating temperatures. 52 and 54of carbon or similar material capable of withstanding high temperaturesand reciprocating with a mini mum of lubrication ride on the guidemembers Si) and'- Spaced shoe or bearing rings cient volume to cool thatportion of the piston between the shoe rings 52 and 54 if desired, beingdrained from the interior of the tubular guide member 50 in a mannerwell known in the art. For this purpose the rings 52 and 54 are drilledaxially.

The piston being supported and guided on the tubular member 50, thereneed be no contact whatever between the piston and the cylinder in whichit operates. Thus piston rings are dispensed with and no lubrication ofthe cylinder walls is used. In the crown or forward portion of the powerpiston 20, the interior walls are insulated as at 62 where in othermachines this portion is cooled either by air or by circulating oil. Anyoil circulated into the piston through the ports 60 in the presentmachine thus performs no cooling function so far as the piston crown isconcerned. The exterior portion of the crown is spaced from the cylinderwall by a circumferential clearance, and can operate at the meantemperature of the combustion cycle.

g The circumferential clearance around the power piston crown allows fora certain amount of leakage of gas from the high pressure part of thepower cylinder cycle directly to the turbine inlet pressure levelwithout supplying its expansion energy to the compressor piston. Thisenergy loss is partly regained in the resulting temperature rise in thegas llow to the turbine, and it will be noted that none of the gasleaves the ow pattern of the machine. Whatever leakage occurs from thepower cylinder in the direction of the scavenging ports 36 merelyincreases the scavenging pressure available when these ports are nextopened.

It has been found that excellent sealing of the power cylinder, or lowleakage, can be obtained if the exterior of the power piston is made inthe form of an elongated labyrinth seal. That is, if the exterior of thepiston is formed with a series of circumferential grooves 65 of theforms shown in Figs. 2, 3 and 4. Of these groove forms, the preferredone appears at present to be that shown in Fig. 3. In this form theleakage velocity past the crest of the groove is high and the gasexpands into the base 66 or larger volume of the groove with someturbulence. In cross section the groove slopes gently away from itscrest in the direction of gas ow, but the rear wall is either undercutas in Fig. 3 or radial as in Fig. 4 so that gases expanding into thegroove having passed the crest are baffled and must make a substantialchange in direction before flowing out of one groove into the next. Therear wall of each groove thus makes an angle of at least 90 with respectto the axis of the piston.

This passage and expansion of the gas is repeated from groove to groovein the series and the specific leakage of gas during the high pressurepart of the cycle is low. It has been found that the loss of energy inthe cycle due to leakage is much less than the gain in energy due toreduction in cooling losses so that there is a net gain in eiciency overa conventional cooled unit. Tests indicate that in a typical smallmachine a running clearance of .020 all around the power piston resultsin leakage losses of approximately the same value as the eliminatedcooling losses, so that any lesser clearances give rise to an increasein thermal eticiency over a cooled machine with piston rings. The sealarea formed by the grooves must at least equal the length of stroke ofthe pistons. Thus if the machine operates with a nominal l inch stroke,the front 10 inches of the power piston at least should be formed withthe labyrinth seal grooves.

It is, of course, understood that in a free piston machine the strokelength varies from part load operation to full load operation.

The power cylinder is insulated as at 67 and the exhaust port header ormanifold is also insulated as at 68.

In some instances it may be desirable to eliminate piston rings from thecompressor piston also. This can be done since the compressor piston(being integral with the power piston) is supported by the centraltubular guide member 50. A seal ring 69 of carbon or the like can besupported on the periphery of the compressor piston and serve to isolatethe compressor cylinder from the bounce cylinder. A labyrinth seal onthe periphery of the compressor piston may alsobe used.

The shank or trunk portion 22 of the piston which connects the power orcrown portion 20 with the compressor piston 24 can be cooled if desiredto reduce the quantity of conducted heat into the compressor piston.

If cooled, for example by oil owing through the dlstributor ring 56,this portion should be of substantially smaller diameter than the hotcrown or power portion to reduce heat transferby radiation from the hotcylinder wall during the inner portion of the stroke.

It will thus be seen that the present invention provides a free pistonpower gas generator in which lubrication is confined to only a smallexterior surface of a single piston supporting and guide member, and thepower piston and its cylinder operate dry and at the mean temperature ofthe combustion cycle. If satisfactory dry carbon or other bearings aredeveloped even the lubrication of the guide 50 may be dispensed with.Thus the quantity of lubricating oil used is greatly reduced, and theadverse effects of carbonized or deteriorated oil entirely eliminated.

While the invention has been disclosed in connection with a particularform and disposition of the parts, it should be expressly understoodthat it is capable of numerous modifications and changes withoutdeparting from the scope of the appended claims.

What I claim is:

l. In a free piston power gas generator, the combination of a frame,aligned power, compressor and bounce cylinders carried by said frame,connected power and compressor pistons working in said cylinders, saidpower piston having a series of peripheral sealing grooves extendingfrom its tiring end, said peripheral sealing grooves increasing incross-sectional area in the direction of gas leakage and having rearwalls disposed at least from the axis of the piston, a body ofinsulation disposed within said power piston radially opposite to saidperipheral sealing grooves, and means carried by said frame to supportat least the grooved portion of said power piston entirely out ofcontact with its respective cylinder.

2. The combination of elements defined in claim l in which the rear wallof each of said peripheral sealing grooves is undercut with respect tothe crest of the next succeeding groove.

References Cited in the tile of this patent UNITED STATES PATENTS2,645,213 Huber July 14, l953 2,671,606 Ricardo Mar. 9, 1954 FOREIGNPATENTS 605,191 Great Britain July 19, 1948

